The present invention relates generally to a method of protecting articles having a bare ferrous base surface and, more particularly, to a method wherein a thermosetting powder is electrostatically applied to a galvanized metal layer covering a bare ferrous based substrate.
Parking meter housings include several different parts made of metal based material. To enable the parking meter housing parts to withstand the abuse to which they are subjected, the parts are usually made of cast iron or zinc, and electrostatically powder coated. Housings of other outdoor token accepting devices, such as consoles for accepting credit and debit cards for several adjacent parking spaces, include parts having a steel base. These token accepting devices are subject to many adverse conditions, such as vandalism and harsh environmental conditions. The vandalism is frequently of the petty type, such as cutting the housing surface with a knife. The harsh environmental conditions include oxidation, leading to rusting of housing parts having a ferrous base, and/or hydrocarbon deposits resulting from automotive vehicle tailpipe emissions, for example. Because of these adverse conditions, parking meters housings and other outdoor token accepting devices generally must undergo substantial maintenance after only five to seven years of use. However, the maintenance usually only includes spray painting the housings, which does not put the meters and other token handling devices back into anything resembling new condition.
Officials of several jurisdictions have also decided that parking meters, and the like, can be used for beautification purposes. For example, officials in seashore communities have decided parking meter housings should have a teal color to complement the color of the adjacent sea. Other communities have decided the usual battleship gray color of parking meter housings is inappropriate and have decided the housings should be a silver like color. The expectation is that an attractive color for parking meter housings and the housings of related devices will be attractive to visitors and might reduce petty vandalism.
I am aware of the prior art disclosing the combination of a ferrous substrate, a galvanized coating on the ferrous substrate and an organic powder coating over the galvanized coating. For example, Geary et al., U.S. Pat. No. 4,540,637 indicates a steel surface must be thoroughly cleaned of dirt, oil, oxidation products and other foreign matter prior to the deposition of a powder coating on the steel. Geary et al. also discloses applying such coatings over mild or xe2x80x9cblackxe2x80x9d steel. The ""637 patent also states surface preparation systems have been developed for mild steel. The preparation systems typically include first cleaning the steel and sometimes roughening it, then rinsing the steel with a solution which deposits a microscopic layer of crystalline material, such as zinc phosphate, on the steel. The microscopic layer passivates the surface against corrosion and provides bonding sites for the powder. Geary et al. specifically states that hot dipped or electroplated galvanized coatings on steel substrates are not normally recommended as a base for thermosetting organic powder coatings. This is so despite the superior resistance to corrosion provided by the zinc cladding of the galvanized layer. Geary et al. states previous experience has shown the thermosetting organic powders do not bond as well to the inherently smooth zinc cladding as to properly prepared mild steel.
This problem was previously recognized by Wlodek in U.S. Pat. No. 3,674,445. Wlodek attempts to solve the problem by vacuum vapor depositing zinc onto the steel substrate. As Geary et al. points out, vacuum vapor depositing zinc is an exotic technique which produces microscopically rough surfaces as compared to galvanized steel. Vacuum vapor deposition can only occur in vacuum processing facilities at very low pressures, on the order of 10xe2x88x925 Torr or less. Such processes are not generally suitable for large scale deposition on large products, but are generally used extensively in processing of small articles, such as integrated circuits.
Geary et al. points out that the problem of depositing a thermosetting organic powder on a zinc coating covering a ferrous base has previously been dealt with by Gemmer in U.S. Pat. No. 3,090,696 and Wamant et al., UK Patent 815 756, both of which say coating adhesion is improved by roughening the surface prior to applying the coating. In Wamant et al., xe2x80x9canchor cavitiesxe2x80x9d are created on the surface to achieve good adherence. Bannister, in UK Patent 1 009 055, recommends blasting the surface to be coated with abrasive particles, such as steel shot, to clean the surface and, presumably, to roughen it.
Geary et al. deals with this problem by avoiding the roughening steps through a four-step pretreatment process, including immersing a ferrous workpiece into an acidified rinse including chromium compounds. However, the use of a substantial amount of chromium compounds is inadvisable in large scale manufacturing processes because of the environmental problems associated therewith.
It is, accordingly, an object of the present invention to provide new and improved housings for parking meters and for other similar metal based token accepting devices which are primarily in an outdoor environment.
Another object of the present invention is to provide a new and improved method of protecting metal based parts of parking meter housings and housings of other related token accepting devices.
A further object of the invention is to provide a new and improved method of refurbishing metal based parts of parking meter housings and the housings of other token accepting devices which are primarily used outdoors.
A further object of the invention is to provide a new and improved parking meter housing which is able to withstand vandalism to a greater extent than existing parking meter housings and is less subject to adverse effects of the environment, such as oxidation and/or deposition of organic compounds on the housing.
In accordance with one aspect of the present invention, the problems with the prior art processes are obviated by centrifuging parking meter and other outdoor token accepting device parts while a hot dipped zinc layer is still in a molten state on a clean, bare ferrous base or substrate of the part. The meter parts are then processed the same as in conventional galvanizing operations, i.e. washed in HCl, then pickled in a Znxe2x80x94Pb bath including some Al and then washed in a water bath including some chromate. The galvanized parts then dry, perhaps for many days, but for considerably more than six hours, before being electrostatically powder coated with a non-epoxy organic, electrical insulating electrically charged thermosetting powder. The powder is applied directly to the thus treated galvanized layer on the cast iron substrate, without requiring the use of phosphoric acid which forms and randomly distributes small amounts of zinc phosphate crystals over the substrate surface prior to application of the powder. Hence, certain problems of the Geary et al. patent are avoided.
By centrifuging the parking meter parts under these conditions, excess zinc is removed from the parts to form a somewhat smooth and somewhat rough zinc surface covering the substrate. Such a surface enables a non-epoxy, organic electrical insulating, electrically charged thermosetting powder to adhere to the oppositely charged zinc layer covering the bare ferrous substrate. The powder is then heated to a molten state and cooled. The organic powder cannot be an epoxy because epoxy powders do not have the ability to withstand the harsh environmental conditions to which parking meters and other similar outdoor metal based consoles are exposed.
It has been found, through actual use and experimentation, that excellent adherence properties are attained as a result of the foregoing process. Theoretical analysis leads to the conclusion that a parking meter or other suitable outdoor token accepting device having a ferrous substrate on which a zinc layer is deposited and covered by a non-epoxy, organic, electrical insulating, thermosetting, powder coating has a life expectancy of 60 to 70 years. If the housing is vandalized, for example, by a knife, the galvanized layer xe2x80x9cgrowsxe2x80x9d into the cut created by a knife, to preserve the superior corrosion resistant characteristics of the housing part of the parking meter or other similar outdoor token accepting device.
The powder is preferably selected from the group including polyurethane, acrylics and nylon to form the second coating with a thickness in the range of about 2.5 to 5 mils. If the powder coating thickness is less than about 2.5 mils, the zinc layer is not adequately covered; if the powder coating thickness exceeds about 5 mils, excessive amounts of powder are used to increase powder cost unnecessarily and likely cause flaking of the powder. To assist in providing proper powder coating adherence the zinc coating has a somewhat rough and somewhat smooth surface.
Another aspect of the invention relates to a method of refurbishing an article having a metal base carrying coatings including rust and hydrocarbons. The method includes removing the coatings by brush blasting materials in the coatings to expose the metal base so it is clean and dry. Then a non-epoxy thermosetting electrically insulating organic powder is powder coated on the clean and dry exposed metal base by applying opposite polarity DC charges to the powder and the base and heating the powder while the charges are applied so the powder adheres to and covers the clean and dry exposed metal base.
A further aspect of the invention relates to a method of protecting articles having a clean and exposed bare ferrous base. The method comprises applying a zinc coating to the clean and exposed bare ferrous base by hot dipping the article. The article is then centrifuged while the dip is still hot to remove excess amounts of the zinc coating. The zinc coating thereby has a somewhat smooth and somewhat rough surface. Then a non-epoxy thermosetting electrically insulating organic powder coating is applied to the somewhat smooth and somewhat rough zinc coating surface by applying opposite polarity DC charges to the powder and the surface. The powder is heated while the charges are applied so the powder adheres to and covers the somewhat smooth and somewhat rough zinc coating surface.
The method is, in one embodiment, used to refurbish the articles. The articles, prior to the refurbishing, have undesirable coatings on the ferrous base. Materials in the undesirable coatings are loosened, by burning, from the ferrous base, to provide loose and flaked particles of the undesirable coating materials on the article surface. The loose and flaked particles are mechanically displaced from the article surface so a bare surface of the ferrous base is clean and exposed. Preferably, the loose and flaked particles are removed from the article surface by grit brush blasting with a mixture of different sizes of aluminum oxide grit having different sizes, optimally in the range of 24 to 30 gauge.
When the articles are cast iron parts of parking meter housings or steel parts of token accepting device housings subject to abuse and adverse environmental conditions, the parts are machined for correct tolerances to enable the parts to fit properly together during is reassembly, after the undesirable coatings thereon have been removed and before the parts are zinc coated.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.